Remove non-LTS Node.js versions from Travis CI configuration (closes #19

)

.travis.yml

@@ -1,9 +1,8 @@
 language: node_js
 node_js:
-  - '9'
   - '8'
-  - '7'
-  - '6'
+  - '10'
+  - '12'
 
 install:
   - npm install
@@ -24,5 +23,5 @@ deploy:
     secure: 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
   on:
     branch: master
-    node: 9
+    node: 12
     tags: true